Isolated direct-current (DC) to DC switching voltage converters use a transformer to convert power from an input source into power for an output load. Such voltage converters include primary-side power switches that convert DC input power into alternating current (AC) power that is fed to the primary side of a transformer. AC power supplied on the secondary side of the transformer is rectified to convert it back into DC power which, in turn, is provided to the output load. The primary-side power switches are typically controlled by pulse-width modulated (PWM) waveforms. A PWM controller generates the PWM waveforms with a frequency and duty cycle that are appropriate to meet the power needs of the output load.
The transformer in an isolated DC-DC voltage converter must be protected in order to prevent saturation of the transformer core and associated failure of the primary-side power switches. As the magnetic flux density within the transformer core approaches a saturation level, the external magnetic field can no longer efficiently increase the magnetization of the transformer core. The effect of this is that the primary winding of the transformer begins to appear as an electrical short circuit, which leads to excessive current through the primary winding. Such excessive current also flows through the primary-side power switches and can damage them. In order to avoid destroying the primary-side power switches and other problems that are associated with transformer core saturation, e.g., excessive heat, transformer core saturation must be prevented in DC-DC voltage converters.
The most direct technique for preventing core saturation is to measure the current flowing through the primary-side winding, and compare it against some current limit that indicates the core is saturating. If the primary-side current exceeds this limit, the PWM waveforms may be adjusted (e.g., a duty cycle of a PWM waveform may be reduced) to prevent the core from saturating. This technique requires sensing the primary-side current, which requires additional circuitry and which typically has some associated power loss. While such a technique is effective and may also be used to balance the half cycles of current and flux, it may not be feasible in some applications. In particular, this technique is not appropriate in isolated DC-DC voltage converters wherein the PWM generator and/or controller is located on the secondary side of the transformer.
Without active prevention of core saturation, as described above, a transformer core is susceptible to flux walkaway, in which small mismatches in the positive and negative half cycles of the voltage converter lead to a gradual increase in the flux magnitude which, eventually, leads to saturation of the transformer core. This can be addressed by balancing the positive and negative half cycles. One technique for doing so is to couple power into the primary-side winding using capacitors. This is often not a preferred technique, due to the added circuitry (size), component cost, power loss, etc., as well as that such a technique does guarantee safe operation. Other flux-balancing techniques, which may operate in conjunction with or as alternatives to capacitor usage, serve to balance positive and negative half cycles over a fairly large time scale, but do not immediately recognize and prevent core saturation. Because such flux-balancing techniques are fairly slow acting, the average transformer flux will tend to rise to positive values or fall to negative values, before the flux-balancing techniques are able to compensate for such excursions. In order to ensure a transformer core does not saturate when using such slow-acting flux balancing, the transformer must be designed with a significantly higher flux saturation level than would be required if perfect flux balancing were available. This may be done by choosing a transformer having an air gap or that is physically larger (e.g., having a larger core cross-sectional area) than otherwise necessary, in order to achieve an adequate safety margin for the flux saturation. Such an overdesigned transformer should be avoided in most applications, due to, e.g., its increased size and increased cost.
Accordingly, there is a need for improved techniques for estimating magnetic flux within the transformer of a DC-DC voltage converter, for immediately preventing saturation of the transformer core, and for balancing the positive and negative flux excursions within the transformer. Such techniques should be feasible for implementation on the primary or the secondary side of the voltage converter, and should not require sensing the primary-side current. Use of such techniques should allow isolated DC-DC voltage converters to be designed with smaller and more efficient transformers, while achieving safe operation that avoids transformer core saturation.